Rutulkumar Patel

Radiosensitization of non-small-cell lung cancer cells and xenografts by the interactive effects of pemetrexed and methoxyamine

BACKGROUND AND PURPOSE The anti-folate pemetrexed is a radiosensitizer. In pre-clinical models, pemetrexed is more effective along with the base-excision-repair inhibitor methoxyamine. We tested whether methoxyamine enhances pemetrexed-mediated radiosensitization of lung adenocarcinoma cells and xenografts. MATERIALS AND METHODS A549 and H1299 cells were evaluated for cell cycle distribution by flow cytometry, radiosensitization by clonogenic assay, and DNA repair by neutral comet assay and repair protein activation. H460 cells were included in some studies. Xenografts in nude mice received drug(s) and/or radiation, and tumor growth was monitored by caliper and in vivo toxicity by animal weight. RESULTS Exposure to pemetrexed/methoxyamine for 24 (H1299, H460) or 48 (A549)hours before irradiation resulted in accumulation of cells near the radiosensitive G1/S border; dose-enhancement factors of 1.62±0.19, 1.97±0.25, and 1.67±0.30, respectively; reduction of mean inactivation dose by 32%, 30%, and 46%, respectively; and significant reductions of SF2 and SF4 (p<0.05). Radiosensitization was associated with rapid DNA double-strand-break rejoining and increased levels of DNA-PKcs. Both tumor-growth rate and tumor-growth delay were significantly improved by adding methoxyamine to pemetrexed pre-irradiation (p<0.0001); no mice lost weight during treatment. CONCLUSIONS Addition of methoxyamine to pemetrexed and fractionated radiotherapy may improve outcome for patients with locally advanced non-squamous non-small-cell lung cancer.

Long-Term Deficits in Behavior Performances Caused by Low- and High-Linear Energy Transfer Radiation

Efforts to protect astronauts from harmful galactic cosmic radiation (GCR) require a better understanding of the effects of GCR on human health. In particular, little is known about the lasting effects of GCR on the central nervous system (CNS), which may lead to behavior performance deficits. Previous studies have shown that high-linear energy transfer (LET) radiation in rodents leads to short-term declines in a variety of behavior tests. However, the lasting impact of low-, medium- and high-LET radiation on behavior are not fully defined. Therefore, in this study C57BL/6 male mice were irradiated with 100 or 250 cGy of γ rays (LET ∼0.3 KeV/μm), 10 or 100 cGy of 1H at 1,000 MeV/n (LET ∼0.2 KeV/μm), 28Si at 300 MeV/n (LET ∼69 KeV/μm) or 56Fe at 600 MeV/n (LET of ∼180 KeV/μm), and behavior metrics were collected at 5 and 9 months postirradiation to analyze differences among radiation qualities and doses. A significant dose effect was observed on recognition memory and activity levels measured 9 months postirradiation, regardless of radiation source. In contrast, we observed that each ion species had a distinct effect on anxiety, motor coordination and spatial memory at extended time points. Although 28Si and 56Fe are both regarded as high-LET particles, they were shown to have different detrimental effects on behavior. In summary, our findings suggest that GCR not only affects the CNS in the short term, but also has lasting damaging effects on the CNS that can cause sustained declines in behavior performance.

MMR Deficiency Does Not Sensitize or Compromise the Function of Hematopoietic Stem Cells to Low and High LET Radiation

One of the major health concerns on long‐duration space missions will be radiation exposure to the astronauts. Outside the earths magnetosphere, astronauts will be exposed to galactic cosmic rays (GCR) and solar particle events that are principally composed of protons and He, Ca, O, Ne, Si, Ca, and Fe nuclei. Protons are by far the most common species, but the higher atomic number particles are thought to be more damaging to biological systems. Evaluation and amelioration of risks from GCR exposure will be important for deep space travel. The hematopoietic system is one of the most radiation‐sensitive organ systems, and is highly dependent on functional DNA repair pathways for survival. Recent results from our group have demonstrated an acquired deficiency in mismatch repair (MMR) in human hematopoietic stem cells (HSCs) with age due to functional loss of the MLH1 protein, suggesting an additional risk to astronauts who may have significant numbers of MMR deficient HSCs at the time of space travel. In the present study, we investigated the effects gamma radiation, proton radiation, and 56Fe radiation on HSC function in Mlh1+/+ and Mlh1‐/‐ marrow from mice in a variety of assays and have determined that while cosmic radiation is a major risk to the hematopoietic system, there is no dependence on MMR capacity. Stem Cells Translational Medicine 2018;7:513–520

Mlh1 deficiency increases the risk of hematopoietic malignancy after simulated space radiation exposure

Cancer-causing genome instability is a major concern during space travel due to exposure of astronauts to potent sources of high-linear energy transfer (LET) ionizing radiation. Hematopoietic stem cells (HSCs) are particularly susceptible to genotoxic stress, and accumulation of damage can lead to HSC dysfunction and oncogenesis. Our group recently demonstrated that aging human HSCs accumulate microsatellite instability coincident with loss of MLH1, a DNA Mismatch Repair (MMR) protein, which could reasonably predispose to radiation-induced HSC malignancies. Therefore, in an effort to reduce risk uncertainty for cancer development during deep space travel, we employed an Mlh1+/− mouse model to study the effects high-LET 56Fe ion space-like radiation. Irradiated Mlh1+/− mice showed a significantly higher incidence of lymphomagenesis with 56Fe ions compared to γ-rays and unirradiated mice, and malignancy correlated with increased MSI in the tumors. In addition, whole-exome sequencing analysis revealed high SNVs and INDELs in lymphomas being driven by loss of Mlh1 and frequently mutated genes had a strong correlation with human leukemias. Therefore, the data suggest that age-related MMR deficiencies could lead to HSC malignancies after space radiation, and that countermeasure strategies will be required to adequately protect the astronaut population on the journey to Mars.

How Will the Hematopoietic System Deal with Space Radiation on the Way to Mars

Purpose of ReviewTraveling through deep space raises challenges to biological systems that have not been fully appreciated or addressed. In addition to the lack of gravity, the space environment includes exposure to charged remnants of supernova explosions beyond our solar system that travels with enormous velocities and energies, called HZE (high atomic number Z and energy E) particles and have the potential to disrupt chemical bonds within the human body though ionization. As a process, the collision of charged particles with matter is not new to physicists and biologists on Earth, and we have extensive data on low-linear energy transfer (LET) ionizing radiation from both accidental and deliberate exposures, dating back to the discovery of radioactive isotopes by Madame Curie. One of the primary morbidities associated with radiation exposure is the challenge to the hematopoietic system. The purpose of the current review is to discuss some of the basic tenants of hierarchical tissue systems by elaborating the effects of radiation damage to the hematopoietic stem cell and how terrestrial radiation and space radiation differ.Recent FindingsThe last few decades of research in the field of space radiation, which consists of high-LET ions of 4He, 12C, 16O, 28Si, 48Ti, and 56Fe, and low-LET protons, have shown that there is a significantly more deleterious impact on the hematopoietic system by the high-LET ions compared to protons, X-rays, and γ-rays. Ground-based high-LET radiation experiments have shown not only in vitro and in vivo adverse effects on hematopoietic stem cells, but also that human leukemia can be induced in humanized mouse models.SummaryHigh-LET space radiation is more lethal to hematopoietic stem cells compared to low-LET radiation, but further research is required in order to understand the impact of high-LET radiation on hematopoietic malignancies. Most of the ground-based studies, because of technical difficulties and cost issues, have been carried out at high dose rates with only one ion species at a time. What remains to be clearly described, however, is the potential damage to the hematopoietic system from exposure to the more complex types of radiation at low dose rates that will occur during space travel and how space agencies can sufficiently protect our astronauts.